LU-16042 tests: can not get cache size on Arm64

LU-16042 tests: can not get cache size on Arm64

This fix the test fail on Arm64, the cache size can not be
display on /proc/cpuinfo. And even in the VM and somee
older Arm64 CPU, we can not get the cachesize. So it's
better to fallback to a pre-set value here if we don't get
the cache size.

Signed-off-by: Kevin Zhao <kevin.zhao@linaro.org>
Change-Id: I17ce1d8accc69d1489db2071a2741b3927fff302
Reviewed-on: https://review.whamcloud.com/48030
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>